In a satellite communications system receiving signals originating from a plurality of distinct transmitters, the reception performance depends notably on the capacity to separate said signals so as to be able to extract therefrom the message, the transported digital data. The multiplexing of the signals in time, frequency and code is the distinguishing feature of TDMA technologies, the acronym standing for the expression “Time Division Multiple Access”, FDMA the acronym standing for the expression “Frequency Division Multiple Access”, or CDMA the acronym standing for the expression “Code Division Multiple Access”. Space diversity techniques can also be used, notably and are particularly effective in the case of transmission of signals transmitted by a plurality of transmitters of different positions.
In satellite-based maritime surveillance systems, for example of AIS type, the transmission of messages is organized automatically in cells of about 25 nautical miles. Reception in orbit for a task 2500 nautical miles in diameter implies that the satellite must simultaneously manage nearly ten thousand cells. Moreover, certain zones are particularly active in transmission due to the fact that a large number of transmitters is present therein. It is therefore necessary for the satellite to have the capacity to receive a plurality of distinct incident signals. This issue is essential in relation to the satellite or airborne reception of uncoordinated signals, said signals being transmitted for example by transmitters moving on land or at sea.
The relative position of the transmitter with respect to the receiver has an influence on the reception of the signal, in particular on the phase, on the lag and on the Doppler shift. This relative position is customarily expressed using azimuth, elevation and distance coordinates. Processing based on receiving the signals on several antennas and taking into account the phase, the lag and the Doppler shift allows separation of the signals originating from several transmitters even if the latter are in collision, that is to say if they are received simultaneously in one and the same frequency domain. Thus, it is possible by processing to enhance or to eliminate the contribution of certain signals with respect to others, and therefore thus to improve the separation. The type of processing chosen is essential so as to obtain maximum separation of the incident signals and improve the effectiveness of the transmission system, notably in terms of economy of bandwidth.
In order to optimize the system separation capacity, prior art solutions may be implemented. Thus, it is possible to use directional antennas at the level of the satellite receiver, to increase the number of reception antennas, to increase the number of satellites in the constellation and to apply interference suppression algorithms.
By way of example, the use of a fixed directional or scanning antenna by using analog beamforming techniques, makes it possible to reduce the number of messages received simultaneously for a given spot. The drawback of this solution is a decrease in coverage, giving rise to a longer revisit duration or else requiring a significant number of antennas and/or satellites.
It is also possible to use digital and/or analog processing operations relying on antenna arrays positioned at the level of the satellite allowing separation of the signals received, called spatial separation in the subsequent description.
Spatial separation customarily calls upon techniques based on systems of antennas, such as for example the SRFF systems, the acronym standing for the expression “Single Reflector Focal Feed”, the DRAF systems, the acronym standing for the expression “Dual Reflector Antenna Feed”, and other array stream systems including elements of patch, helix, monopole or dipole types.
Digital processing operations are associated with these systems of antennas. Thus, the reference solution called Spatial Adaptive Filtering (SAF) is based on the use of a direct-transmit antenna array associated with a digital beamforming processing.
The technologies of adaptive filtering encompass a set of techniques such as:
the least squares scheme, designated by the acronym LMS standing for the expression “Least Mean Square” and allowing adaptation of the receiver to a known input signal such as a conditioning sequence or a code customarily designated by the expression “training sequence”;
the optimization of the signal-to-noise ratio so as to allow the suppression of the side lobes and the multiple lobes of the spectrum of the signal received;
the determination of a table of pre-calculated weights applied to the signals originating from transmitters whose position is known;
the use of retro-directional rays, the principle being to form a directional ray toward the transmitters received;
the use of a generalized diagram based on the inversion of the covariance matrix representative of the state of the transmission channel.
In satellite communication systems receiving signals originating from a plurality of transmitters, the issue is to separate long messages of several milliseconds, for example 26 ms, and in contradistinction to the signals transmitted in the form of pulses which possess an intrinsic temporal separation, it is necessary to obtain a spatial separation of transmitted signals overlapping at reception.